1. Field
The described technology relates generally to a fuel cell stack and a fuel cell system.
2. Description of the Related Art
A fuel cell is a device that generates electric power, by electrochemically reacting a fuel (hydrogen or a reformate gas) and an oxidizing agent. The fuel cell directly reacts a fuel (hydrogen or a reforming gas) and an oxidizing agent (oxygen or air), which are continuously supplied from the outside, to produce electrical energy. The oxidizing agent is pure oxygen or air containing a large amount of oxygen, and the fuel may be pure hydrogen or a fuel containing a large amount of hydrogen that is generated by reforming a hydrocarbon fuel (LNG, LPG, CH3OH).
A fuel cell may be classified as a polymer electrode membrane fuel cell (PEMFC) that reacts oxygen and a reformate gas having abundant hydrogen by reformate fuel, or a direct oxidation fuel cell that directly reacts a fuel with oxygen.
A fuel cell system includes a stack for generating electricity. The stack includes multiple unit cells stacked on one another. Each unit cell includes a membrane electrode assembly (MEA) and a separator (bipolar plate). The separator includes passages that supply hydrogen and oxygen to the MEA.
A fuel cell system generates heat by the oxidation/reduction reaction of hydrogen and oxygen. By sustaining the stack at an appropriate driving temperature, the stability of an electrolyte film is secured, and the performance of the electrolyte film is prevented from being degraded. For this purpose, the stack has a cooling passage extending there through. Air or cooling water flows through the cooling passage, thereby removing some of the heat generated within the stack.
When a temperature variation occurs at the inside of the stack, the condensation of water is increased at lower temperature portions thereof. Thus, a flooding phenomenon occurs. When the flooding phenomenon occurs, the movement of fuel or an oxidizing agent is disrupted, reducing the output of the stack.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the described technology, and therefore, it may contain information that does not constitute prior art.